Does small-bodied salmon spawning activity enhance streambed mobility?

Abstract

Female salmonids bury and lay their eggs in streambeds by digging a pit, which is then covered with sediment from a second pit that is dug immediately upstream. The spawning process alters streambed topography, winnows fine sediment, and mixes sediment in the active layer. The resulting egg nests (redds) contain coarser and looser sediments than those of unspawned streambed areas, and display a dune-like shape with an amplitude and length that vary with fish size, substrate conditions, and flow conditions. Redds increase local bed surface roughness (<10 21 channel width, W), but may reduce the size of macro bedforms by eroding reach-scale topography (10 0 -10 1 W). Research has suggested that spawning may increase flow resistance due to redd form drag, resulting in lower grain shear stress and less particle mobility. Spawning, also prevents streambed armoring by mixing surface and subsurface material, potentially increasing particle mobility. Here we use two-dimensional hydraulic modeling with detailed prespawning and postspawning bathymetries and field observations to test the effect of spawning by small-bodied salmonids on sediment transport. Our results show that topographical roughness from small salmon redds has negligible effects on shear stress at the reach-unit scale, and limited effects at the local scale. Conversely, results indicate sediment mixing reduces armoring and enhances sediment mobility, which increases potential bed load transport by subsequent floods. River restoration in fish-bearing streams should take into consideration the effects of redd excavation on channel stability. This is particularly important for streams that historically supported salmonids and are the focus of habitat restoration actions.

Zoogeomorphological behaviours in fish and the potential impact of benthic feeding on bed material mobility in fluvial landscapes

“…Most fish research has focused on redd construction by salmonids (DeVries, ) demonstrating that salmon ( Salmo spp.) spawning can affect bed material characteristics (Kondolf et al, ; Montgomery et al, ), bed permeability and hyporheic exchange (Buxton et al, ), topographic drag (Montgomery et al, ), bed material stability (Gottesfeld et al, ; Hassan et al, ; Buxton et al, ), bedload flux (Hassan et al, ) and bedform generation (Field‐Dodgson, ; Gottesfeld et al, ). A welcome extension to this focus on local effects is found in Fremier et al .…”

“…Most fish research has focused on redd construction by salmonids (DeVries, 2012) demonstrating that salmon (Salmo spp.) spawning can affect bed material characteristics (Kondolf et al, 1993;Montgomery et al, 1996), bed permeability and hyporheic exchange (Buxton et al, 2015a), topographic drag (Montgomery et al, 1996), bed material stability (Gottesfeld et al, 2004;Hassan et al, 2015;Buxton et al, 2015b), bedload flux and bedform generation (Field-Dodgson, 1987;. A welcome extension to this focus on local effects is found in Fremier et al's (2018) attempt to understand the impact of salmonid dispersal and bed destabilisation on long-term fluvial erosion and landscape evolution using a numerical modelling approach.…”

Effect of transect location, transect spacing and interpolation methods on river bathymetry accuracy

“…Most river surveys for smaller projects are still cross-section based (e.g. Hassan et al, 2015). Additionally, some work only requires reach-scale indices, like mean width, depth and wetted areas, which can be derived from bathymetries interpolated using data from select cross-sections (Goode et al, 2013;Marzadri et al, 2014a;Buxton et al, 2015;Tranmer et al, 2015).…”

Foraging Fish as Zoogeomorphic Agents: An Assessment of Fish Impacts at Patch, Barform, and Reach Scales

“…Published studies of the role of fish as zoogeomorphic agents in streams have primarily focused on disturbance caused by salmonids during nest building. This is clearly an important mechanism by which fish can substantially alter sediment conditions, near‐bed hydraulics, and sediment transport (Albers & Petticrew, ; Buxton et al, ; Field‐Dodgson, ; Gottesfeld et al, ; Hassan et al, , ; Kondolf et al, ; Macdonald et al, ; Montgomery et al, ; Moore, ; Moore et al, ; Peterson & Foote, ). We speculate that foraging may be equally, if not more, important as a zoogeomorphic mechanism in UK rivers.…”